hot runners, mixed materials molding

8/3/2019 Hot Runners, Mixed Materials Molding

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HOT

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UNNERS

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HE

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EY

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ULTIMATERIAL

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PPLICATIONS

Multimaterial molding has been familiar to

automotive molders for years. Machines with up

to four injection units can be found throughout the

car lens industry, adding complexity to both mold

and machine. The technology has helped to

increase productivity by reducing assembly time

of multicolored parts. The standard method for

molding multicolored car lenses has pioneered the

multimaterial molding process, extending itsapplication uses to a wide range of other markets.

Multimaterial molding has emerged as a way to

give consumer and industrial products a distinct

look, feel, or functionality. For example, a

multimaterial solution to the ink wearing off your

computer keys might be to mold the letters right into

the plastic keys, using a different plastic material. In

the automotive industry, PA and TPE combinations

have become very common because they eliminate

costly and complicated assembly steps.

The molds and machines for such applications

are more complex than are those for single-

material molds; however, the cost saving achieved

by consolidating or eliminating assembly

procedures makes the technology economical.

In multimaterial applications, parts must either

be transferred from one cavity to the other or core-

back movements (see the following example) open

up cavity space for the material following the

initial injection. Technologies used for cavity

transfer include robots and mold- or machine-

mounted turntables. A new technology for cavity

transfer was introduced at the K-show, where

cavities rotate on a vertical axis in the mold (i.e.,

as opposed to the usual horizontal turning axis).

The cavities can rotate faster due to the lower mass

moved. Core-back technology is typically not ascomplex, but there are limitations to part

geometries that can utilize such technology.

Multimaterial molding would be much more

difficult without the use of hot runner technology.

The use of cold and insulated runners is limited.

Part transfer is simplified if a robot does not have

to deal with a cold runner because parts need to be

moved from one cavity to the next at the same

time. Three plate molds cannot be used to eject

two or more separate cold runners from thestationary half of the mold. In other instances the

melt channels coming from the injection units

may overlap on the way to the cavity gate,

requiring one manifold to host multiple channel

systems for various colors.

Two examples of multimaterial hot runner

technology will now be explored.

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DVANCED

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ECHNOLOGIES

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ARTIN

B

AUMANN


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Society of Plastics Engineers

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M

ULTICOLOR

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PPLICATION

C

OSMETIC

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ART

(12 + 12 C

AVITY

M

OLD

)

A shampoo company sought to differentiate its

bottle from the competition by using two different

colors for the base and the lid of the closure

(Figure 1). The shape of the closure and the risk of

flaking ruled out painting or stickering the parts

after molding. In addition, the parts production

needed to be as cost effective as possible, which

meant keeping mold and machine costs to a

minimum.

Two hot runner manifoldsa 12 drop and a

six drop (Figure 2)had to fit into a tight mold

frame and provide balanced part filling. A vertical

injection unit provided the material for the lid

[polypropylene (PP), 3.1 g], and a centralhorizontal injection unit provided the material for

the base (PP, 8.4 g). A computer simulation

provided the necessary input for the channel

layout, considering optimum diameters for

balanced filling and fast color changes. To achieve

a balanced flow, the outer manifold (12 drop)

features four melt channel level changes and five

different melt channel diameters. The inner, less-

complicated manifold (six drop) has one level

change and four different melt channel sizes. Such

effort is necessary to achieve balanced filling.

Cost optimization was achieved by utilizing

core-back technology. A small moveable core in

the hinge area of the base cavity retracts after the

base material is injected and cooled. The retract

core provides the necessary interface between

base and lid cavity. The lid material is then

injected directly via a hot tip nozzle. This simple

approach avoided the use of a rotary table or a

robot to move the base in order to mold the lid

onto it. As a final step, the two-colored closures

are closed prior to part ejection via a mechanicalcam and are ready for assembly onto the bottle.

M

ULTIMATERIAL

A

PPLICATION

F

ASTENER

PATPE(4 + 4 C

AVITY

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OLD

)

Two hot runners in On top of the other

formation (Figure 3) were required to mold a

small automotive fastener. The part is made up of

two very different materials. A polyamide (PA)

base provides the necessary structural integrity for

the part as well as the fastening function.

Figure 1. Shampoo closure.

Figure 2. Shampoo closure hot runner. Figure 3. On top of the other formation.


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Advanced Technologies

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Thermoplastic elastomer (TPE) material is

molded on to it to provide a waterproof seal. The

4.5-g PA base is directly gated via a hot tip

nozzle. The TPE seal is gated via two valve-gated

nozzles and a short cold runner. Because of the

close pitch spacing (


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Is the Machine Stack Ready?

A stack mold can virtually double the output

of an injection molding machine. The hot runner is

one of the key technologies for all stack

applications. It is responsible for delivering the

melt to the center section of the mold. The

importance of having a stack-ready, optimizedmachine for stack applications is often

overlooked; not every machine is built to accept a

two- or four-level stack mold. The injection

molding machine must be able to accept a mold

that typically has twice the weight and shut height

of a single-face mold that is molding the same

parts. To minimize mold wear, special attention

has to be given to the robustness of the machine.

There are also a variety of options available for

driving and supporting the center section with the

hot runner of the mold.

There are Some Items to Consider:

Clamp Opening.

Stack mold systems with

two mold openings require approximately

double the shut height and opening stroke

relative to single-face applications.

Injection Capability.

The machine must

provide double the shot weight, plasticating

ability, and injection rate than that required

for a comparable, singe-face mold.

Tie Bar Support for Center Section.

Machine tie bars must provide sufficientsupport for the hot runner and cavity plates

as well as accurate, repeatable alignment

during mold open and close to prevent

damage to cores and cavities.

Sprue Break Capability. The injection unit

of the machine must retract from the sprue

bar enough to clear it in order to prevent

collision and possible damage between the

injection nozzle and the sprue bar upon

mold close.

Injection Unit Shut-off Nozzle

. This

allows

recovery and prevents resin drool while the

mold is open. Although it is possible to run

stack molds without it, the shut-off nozzle

will help improve cycle time (recommended).

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ARIETY

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TACK

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OLDING

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EEDS

Standard Stack Hot Runner

The standard stack hot runner features a center

sprue bar for melt delivery to the center section.

The parts produced on both the injection and

clamp sides of the mold are the same resin and

part weight. These systems come available as two-

level molds (two mold faces) or four-level molds

Figure 5. Harmonic linkage (4 16 stack).

Figure 6. Harmonic linkage provides precisewear-free center section movement.

Figure 7. Tie bar support assures minimizeddeflection, decreasing potential mold wear.


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Advanced Technologies

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(four mold faces). A rugged center section

support is key to minimizing wear on cavity and

core shut-offs. The guided sprue bar features an

antidrool bushing to reduce drool. On systems

like this, parts typically freefall from the mold.

Some application examples are: closures,

packaging, and housewares.

Offset Stack Hot Runner

On an offset stack hot runner, the sprue bar isoffset to the side of the system. Offsetting the

sprue bar allows plastic to be fed into the hot

runner at a location other than the center. This is

ideal for large, flat parts. Robot access is

simplified because the offset sprue bar can be

placed where it will not interfere.

Some application examples are: washing

machine components and large houseware

containers.

Family Stack Hot Runner

These specialty stack systems are used to

make related parts in the same mold (e.g., to

produce a lid and base). The challenge with a

family mold is the balancing of the two different

parts for equal filling, which is critical to

producing quality parts. Cavity filling studies are

applied to determine proper filling.

Some application examples are: part

assemblies like the one shown in Figure 3.

Stack Platen

The stack platen is designed to run two

identical, independent, single-face molds in a

single press, thereby doubling the production of the

machine. The attraction for molders is that it allows

Figure 8. Standard stack hot runner

showing center sprue bar.

Figure 9. Stack hotrunner with offset

sprue bar.

Figure 10.Flow pattern for

family stackhot runner.

Figure 11. Stack platen.


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the existing single-face molds to operate in a stack

configuration. This approach permits flexibility

because the molds could also be run independently.

Another benefit of the stack platen is that molds for

different parts can be used in the same carrier aslong as they have similar projected areas.

Some application examples are: any in which

productivity needs to be increased on existing

injection molding machines.

Stack Carrier

The stack mold carrier rides on the machine

bed and has the ability to support very heavy

molds. Because the linkage uses bearings instead

of gears, wear is minimal. This technologyeliminates the need for individual, mold-mounted,

center-section supports. The stack mold carrier

facilitates quick mold changes and automatic part

removal because the mold can be lowered in from

the top and bolted on.

Some application examples are: very heavy

stack molds that require a lot of center section

support.

Figure 12. Stack mold carrier.

hot runners, mixed materials molding

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